Motor vehicle disk brake systems utilize, at each wheel, a brake rotor connected to an axle hub of a rotatable axle of the motor vehicle, and an opposing set of selectively movable brake pads connected to a non-rotating brake caliper which carries a set of brake pads. The brake rotor includes a disk shaped rotor cheek having opposing brake pad engagement surfaces, wherein when braking is to occur, the braking system causes the caliper to press the brake pads upon respective brake pad engagement surfaces of the rotor cheek. Frictional interaction between the rotating rotor cheek and non-rotating brake pads causes braking of the motor vehicle to transpire, the rate of braking depending upon the pressure of the brake pads against the rotor cheek.
In the automotive art, modern dual-circuit hydraulic braking systems for automotive applications typically include an operator-actuated brake actuation unit, such as a tandem master cylinder actuated by a booster-aided brake pedal, by which to supply a first pressurized brake fluid to each of a first pair of wheel brakes via a first or “primary” braking circuit, and a second pressurized brake fluid to each of a second pair of wheel brakes via a second or “secondary” braking circuit. The use of wholly redundant braking circuits for operating discrete pairs of wheel brakes ensures continued vehicle braking capability, notwithstanding a degradation of performance of one of the braking circuits.
In order to achieve an “anti-lock” braking system (ABS), each braking circuit often features a normally-open electrically-operated inlet valve controlling the flow of pressurized fluid to each wheel brake, while a pressure relief line that includes a normally-closed electrically-operated outlet valve, a return pump, and a check valve controls the return of pressurized fluid from the wheel brake to the hydraulic brake line upstream of the inlet valve. A “separation” or “isolation” valve, located in the hydraulic brake line of each circuit upstream of the location at which the pressure relief line connects to the brake line, serves to isolate the brake line from the master cylinder during anti-lock operation.
Increasingly, such anti-lock braking systems are used in combination with wheel speed sensors in a traction control mode. The further addition of a steering angle sensor, a vehicle yaw rate sensor, and a lateral vehicle acceleration sensor in conjunction with vehicle speed, wheel speed, and wheel longitudinal slip enables such anti-lock braking systems to operate in an “electronic stability control” mode, wherein a braking system electronic control unit (ECU) selectively energizes each circuit's electrically-operated valves when the controller identifies an opportunity to enhance vehicle stability through a selective application of the vehicle's brakes.
In order to control the brake fluid pressure in traction control or vehicle stability control modes, a hydraulic pump is typically placed in the pressure relief line of each circuit downstream of the outlet valve to return pressurized fluid to the circuit's hydraulic brake line. The pump also serves to provide an increasing rate of fluid pressure upon the closing of the isolation valve to provide a sufficient braking system response time when operating in a traction control mode, even at a time when the brake fluid has a relatively-high viscosity due, for example, to low brake fluid temperatures.
The prior art has recognized, however, that a quicker system response is desirable when the braking system is operated in a vehicle stability control mode. By way of example, a rapid pressure build up in one or the other braking circuit is particularly desirable upon commencing vehicle stability control in order to correct oversteer or understeer conditions. Accordingly, the prior art teaches the addition of a braking circuit pre-charging function to the brake actuation unit, i.e., to the vacuum booster of the master cylinder, in order to increase system response at the time such vehicle stability control is commenced. Alternatively, an additional pre-charging pump is provided in one or both braking circuits to ensure a sufficient increasing rate of fluid pressure at the commencement of vehicle stability control enhancement.
There are multiple Electronic Stability Control (ESC) system implementations on the road today. Although all of them attempt to perform the same task of helping the driver retain reasonable directional control under nonlinear vehicle dynamic conditions, these ESC systems have some distinct implementation differences and can be divided into four categories as defined and described in The Society of Automotive Engineers (SAE) Surface Vehicle Information Report, SAE J2564, “Automotive Stability Enhancement Systems”, revised June, 2004 and superceding version issued December, 2000.
Elements that ESC systems have in common include ABS and the ability to sense steering wheel position; the ability to calculate vehicle speed; the ability to sense yaw velocity and lateral acceleration; and the ability to build and control braking force in the channels used for yaw stability control independent of the driver's input to the vehicle braking system. An example of the implementation of a vehicle hydraulic braking system utilizing an ESC system is described in U.S. Pat. No. 6,896,338 B2 to Nakayasu et al., which patent is hereby incorporated herein by reference in its entirety.
Referring now to FIGS. 1A and 1B, the structural and operational aspects of a prior art anti-lock braking system (ABS) will be described, keeping the description limited to those portions thereof having relevance to the present invention.
An ABS 10 includes an electronic control unit (ECU) 12 which is electronically interfaced with a brake pedal assembly 14, at least one hydraulic brake fluid pump 16, and at each wheel with an inlet valve 18, and an outlet valve 20. An hydraulic brake line 22 is interfaced with the brake fluid pump and the inlet and outlet valves at each wheel, and is further interfaced with a master cylinder brake fluid reservoir 24, and still further interfaced at each brake corner (i.e., at each vehicle wheel) with caliper actuators 26 (consisting of one or more cylinders 26a and pistons 26b). A brake caliper 28 is non-rotatively affixed at each wheel in straddling relation to the brake rotor 30 of the respective brake corner (which is, in turn, connected in fixed relation to the rotative wheel axle (not shown). In a braking system utilizing a sliding brake caliper (as is shown in FIGS. 1A and 1B), one side 28a of the brake caliper is hydraulically active and the other side 28b hydraulically inactive. In a braking system utilizing a fixed brake caliper, both sides of the brake caliper are hydraulically active. In either case, a brake pad 32a, 32b is respectively affixed at both sides of the brake caliper, so that when the hydraulic brake fluid in the caliper portion 22a of the brake line 22 is pressurized, the brake caliper causes the brake pads to squeeze upon the cheeks 30a of the brake rotor 30.
In operation, the ECU constantly runs predetermined algorithms while receiving vehicular behavior inputs (via sensors) to determine brake action commands. In FIG. 1A, a brake apply situation is occurring. The ECU has commanded the inlet valve be open, the outlet valve be closed and the brake fluid pump to rapidly energize so as to provide a high pressure HP brake fluid in the caliper portion of the brake line, thereby applying the brakes in the sense that the brake pads press hard against the rotor cheeks. The normally open inlet valve and normally closed outlet valve are such during normal braking, being selectively opened and closed by the ECU during an ABS event, as for example when slip at the wheels is occurring. In FIG. 1B, a no brake apply situation is occurring. The ECU has commanded the inlet valve to remain open, the outlet valve to remain closed and the brake fluid pump to de-energize so as to provide a low pressure LP brake fluid in the caliper portion of the hydraulic brake line, thereby releasing the brakes in the sense that the brake pads no longer press hard against the rotor cheeks.
The focus of current braking system design is on rapid response to reduce vehicle stopping distance and on high pressure sealing to ensure hydraulic brake system integrity. These design targets typically require a sealing implementation in which the brake pads are unable to actively retract from the brake rotor after brake application, since the response of the braking system is optimized for quick response and high sealing ability.
In this regard, the application of the brake pads to the rotor flexes the seals of the hydraulic braking system. Since the braking system is designed for quick response and sealing of high hydraulic pressures, the seals do not fully return the braking system back to zero pressure, and the brake pads remain in adjacency with the brake rotor cheeks even when the brake pedal is not pressed by the vehicle operator. Thus, the brake pad material is kept in close contact to the rotor cheeks in all conditions of the motor vehicle.
This close contact of the brake pads with the brake rotor during brake rotor rotation creates a frictional force due to the residual force of the seal (from the last brake application) which, in turn, creates a torque in the opposite direction of vehicle's forward rotation. This torque, termed drag torque, reduces the efficiency of the vehicle's propulsion system and thereby increases fuel consumption. When the vehicle is parked, this same residual force from the seals keeps the brake pad material in contact with the brake rotor cheeks. When these components, which are typically manufactured with a percentage of metal, are exposed to moisture, they can corrode via galvanic action. This galvanic action creates inconsistencies on the rotor cheeks and brake pad surfaces locally on the section of the brake rotor where these components were in mutual contact. These surface inconsistencies create a local rotor cheek thickness and surface material property mix different than the rest of the rotor cheeks. This physically unique section of the brake rotor creates a different frictional force compared to the rest of the brake rotor during rotation when the brakes are applied. This varying frictional force creates a periodically varying frictional torque, the period coinciding with the rate of rotation. This variable torque excites the brake caliper which, in turn, excites the suspension components of the vehicle, and this resultant vibration is sensed by the vehicle's operator through the steering wheel and the body, and is also sensed as pulsation of the brake pedal. The vehicle operator can perceive this condition as annoying and may seek early brake servicing.
The aforesaid problems associated with current brake pads retraction modalities were solved by an active brake pads retraction system and method disclosed in U.S. patent application Ser. No. 11/739,721, filed Apr. 25, 2007 to inventors Leach, et al. and assigned to the assignee hereof, which application was published as U.S. Patent Application Publication 2008/0265663 A1 on Oct. 30, 2008, the disclosure of this patent application and patent publication being hereby incorporated herein by reference in entirety (hereinafter referred to as the “GM Active Brake Pads Retraction System”.
The GM Active Brake Pads Retraction System discloses an active brake pads retraction system which retracts the brake pads with respect to the brake rotor responsive to predetermined conditions of the motor vehicle in which braking is not required. To accomplish this benefit, the GM Active Brake Pads Retraction System teaches selective application of negative hydraulic brake line pressure to cause the caliper pistons to retract with respect to their respective caliper cylinders, thereby causing the brake pads to retract from the brake rotor (i.e., the brake pads are affirmatively relocated by the brake line vacuum differential pressure with respect to atmospheric into a spaced relation with respect to the rotor cheeks.
According to the GM Active Brake Pads Retraction System, the electronic and hydraulic brake control system of current anti-lock brake systems (ABS) are utilized, wherein these control systems are uniquely configured to provide active retraction of the brake pads from the brake rotor during predetermined conditions of the motor vehicle when braking is not required, as for example when the motor vehicle is cruising or parked.
In operation of the GM Active Brake Pads Retraction System, the electronic control unit (ECU) of the ABS constantly runs algorithms, including an active brake pad retraction algorithm, while receiving vehicular behavior inputs to determine brake action commands. Upon command from the ECU to execute active brake pads retraction, the normally open inlet valve of the hydraulic brake line is closed and the normally closed outlet valve of the brake line is opened. These valve settings isolate the caliper portion of the brake line from the pressure side, and simultaneously expose it to the vacuum side, of the hydraulic brake fluid pump. The brake fluid pump is thereupon immediately energized, pushing the hydraulic brake fluid in the brake line upstream of the brake fluid pump towards the master cylinder reservoir, which is vented to atmosphere. The brake line downstream of the brake fluid pump is therefore exposed to a negative (i.e., below atmospheric) brake line pressure. This negative pressure with respect to atmospheric pressure applies a suction-retraction force to the caliper portion of the brake line. This pressure differential is registered at the caliper pistons, which are caused to be pulled in the direction of the negative pressure to compensate for the displacement of the brake fluid pumped into the master cylinder brake fluid reservoir on the pressure side of the brake fluid pump. This displacement of the caliper pistons actively moves the brake pads, which are affixed to the pistons, as for example via brake pad clips known in the art, away from the rotor cheek surfaces.
According to the GM Active Brake Pads Retraction System, in a braking system utilizing a sliding brake caliper, one side of the brake caliper is hydraulically active and the other side is hydraulically inactive, wherein the hydraulically active side brake pad tends to retract first until the brake caliper reacts to allow the hydraulically inactive side brake pad to also retract. Further according to the GM Active Brake Pads Retraction System, in a braking system utilizing a fixed brake caliper, both sides of the brake caliper are hydraulically active and the caliper pistons retract independently of each other as both are directly affected by the negative pressure in the caliper portion of the brake line. Finally according to the GM Active Brake Pads Retraction System, in either type of brake caliper, brake pad retraction creates a gap between the brake pads and the brake rotor, thereby reducing parasitic brake drag and galvanic corrosion.
While the GM Active Brake Pads Retraction System solves the problems associated with current brake pads retraction modalities, what remains needed is some way to positively limit the maximum retraction distance of the brake pads with respect to the rotor cheeks.